In the following sentence improvement question, choose the alternative that best replaces the underlined conjunction so that the sentence is grammatically correct: "Though he worked hard, but he failed."

Introduction / Context:
This question tests your understanding of conjunctions and the correct way to join contrasting clauses. The sentence uses "though" at the beginning and then adds "but," creating a common error of double contrast. Examinations frequently check whether you can recognise and correct such redundancy.

Given Data / Assumptions:

• Original sentence: "Though he worked hard, but he failed."
• There are two clauses: "he worked hard" and "he failed."
• "Though" already indicates contrast.
• You must select the conjunction that keeps the contrast but removes the redundancy.

Concept / Approach:
In English, you do not usually combine "though" and "but" in the same sentence to express a single contrast. You can say "Though he worked hard, he failed," or "He worked hard, yet he failed," but not "Though he worked hard, but he failed." One correct pattern is "Though ... yet ..." which is commonly accepted in exam English.

Step-by-Step Solution:
Step 1: Identify that "though" already expresses a concessive contrast. Step 2: Recognise that "but" after "though" creates unnecessary double contrast. Step 3: Among the options, "yet" appropriately completes the correlative pattern "though ... yet." Step 4: Rewrite the sentence as "Though he worked hard, yet he failed." Step 5: Confirm that the sentence now expresses a clear contrast in a grammatically acceptable way.

Verification / Alternative check:
You can also check by removing the initial "though." If you say "He worked hard, yet he failed," the sentence is perfectly correct. This shows that "yet" is the right contrastive conjunction here. In many grammar books, "though" and "yet" are listed as a correlative pair, which supports this choice for standard exam usage.

Why Other Options Are Wrong:

• and: "Though he worked hard, and he failed" does not show contrast and makes the sentence awkward; "and" is a coordinating conjunction of addition, not concession.
• then: "Then" mainly indicates time or consequence, not contrast, so it does not fit the concessive meaning.
• No improvement: Keeping "but" after "though" preserves the redundancy and the grammatical error.

Common Pitfalls:
A very common mistake is to overuse "but" after words like "though," "although," or "even though." Learners feel that the contrast is not complete without "but," but in fact "though" itself already introduces the contrast. Remember either "Though he worked hard, he failed" or the correlative "Though he worked hard, yet he failed," but avoid using both "though" and "but" together in this structure.

Final Answer:
The correct improvement is yet, giving the sentence: Though he worked hard, yet he failed.